[petsc-dev] What's the point of D(A/M)GetGlobalVector?

[Aron Ahmadia]

Thanks Matt and Jed,

I think I'm straight on usage/philosophy here.

A

On Fri, Aug 27, 2010 at 2:42 PM, Matthew Knepley <knepley at gmail.com> wrote:

> Simply, in PETSc, getFoo() and restoreFoo() operate an object pool.
>
>    Matt
>
>
> On Fri, Aug 27, 2010 at 11:23 AM, Jed Brown <jed at 59a2.org> wrote:
>
>> On Fri, 27 Aug 2010 14:13:01 +0300, Aron Ahmadia <
>> aron.ahmadia at kaust.edu.sa> wrote:
>> > What exactly is the purpose of these routines then?  Is there a global
>> > Vector associated with a DA?  If so, why are the values uninitialized?
>>
>> It's common to need work vectors in places like residual evaluation and
>> Jacobian assembly.  There is a little bit of setup cost to allocate a
>> new vector each time, so usually we'd prefer that they be persistent and
>> just reuse them.  One option would be to make the user manage this
>> themselves, but that's error prone because it's easy to accidentally
>> alias the work vectors, so instead the DA keeps a cache of vectors.  It
>> starts out empty, and each time you call DAGetGlobalVector(), the cache
>> is searched for an available vector.  If none are found, a new one is
>> allocated and the cache grows by one.  DARestoreGlobalVector() checks a
>> vector back in so it may be used elsewhere.  These vectors are destroyed
>> in DADestroy().
>>
>> Jed
>>
>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
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